The present invention relates to new and improved polyurethane adhesive compositions. More particularly, it relates to one package polyurethane adhesives based upon poly aliphatic isocyanate compounds which are storage stable for prolonged periods of time and which are curable by heat to form high bond strength adhesive joints which exhibit excellent adhesion to a variety of substrates.
In this application, the term "aromatic isocyanate" refers to an organic isocyanate compound wherein the isocyanate group(s) is bonded directly to a carbon atom of an aromatic nucleus. By "aliphatic isocyanate" is meant an organic isocyanate compound wherein the isocyanate group or groups are bonded directly to an aliphatic carbon atom. By "polyaliphatic isocyanate" is meant an aliphatic isocyanate having more than one such isocyanate bonded to aliphatic carbon atoms in the molecule.
Polyurethane adhesives are widely used for bonding various metallic, plastic and glass substrates because they form good adhesive bonds with these substrates. Generally, prior art polyurethane adhesive compositions comprise two component systems in which a diisocyanate-containing component and a polyol containing component are properly measured and mixed immediately prior to applying the adhesive to the substrate. These compositions have extremely rapid gellation times upon standing at room temperature of a few minutes to a few hours. After gellation, the compositions cannot be further used as adhesives because they are difficult, if not impossible to apply and do not bond satisfactorily to substrates.
In U.S. Pat. No. 4,119,594, thermosetting polyurethane compositions having improved pot lives useful for forming injection molded articles are disclosed. These compositions comprise diphenylmethane diisocyanate or a liquid isocyanate-terminated reaction product of diphenylmethane diisocyanate and a diol; about 40 to 85 equivalents of hydroquinone di-(beta-hydroxyethyl) ether, a liquid polyol having hydroxy functionality of greater than two and an equivalent weight of about 2,000 and a catalytic amount of zinc stearate, said composition having an excess of isocyanate groups over hydroxyl groups of up to 10%. The compositions are described as stable at room temperature for up to several hours and cure upon injection molding at temperatures of from about 120.degree. to 170.degree. C. to form satisfactory molded articles.
It has now been discovered that one package heat curable polyurethane adhesive compositions exhibiting good long-term storage stability in a closed container and useful pot lives at room temperature of up to about six months are provided by a mixture or a prepolymer solution of secondary or tertiary polyaliphatic isocyanate compounds and a stoichiometric amount of a diol, polyol, or a mixture of diols and polyols. The adhesive compositions upon heat curing form strong adhesive bonds or joints for a variety of substrates and the inconveniences associated with the measuring, mixing and use of the prior art two-component systems are avoided.